Before the first appearance of trilobites
"If my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown periods of time, the world swarmed with living creatures." - Charles Darwin*
The sudden occurrence of animal fossils in sedimentary rocks of the Lower Cambrian has always been and still is a hotly debated matter, sometimes even a battlefield of contradicting ideologies. Originally, there may have been good reason to call this event the “Cambrian Explosion”, an expression which should be used but with care today. For not only is there growing scientific evidence that its early conclusions and significance have to be looked at in a different light, perhaps even revised. The term is also increasingly abused by certain pseudoscientific movements as a tool to decry not only the theory of evolution but evolution itself. The more or less simultaneous appearance of various forms of life in the fossil record is happily used by many religious groups to involve a supreme being who created this faunal diversity almost in an instant.
Regardless of being a religious person or not – denying evolution in spite of the huge amount of scientific evidence to support it proves to be nothing but sheer ignorance. There is no serious argument amongst the scientific community and the enlightened public that life on earth has been in existence for more than the last 2 billion years, as a matter of fact there are strong indications that life started way before that, perhaps as early as 1 billion years after the formation of the planet, which would set the mark at roughly 3.5 billion years before the present day. But we do not need to look back that far. Our first an foremost interest is in trilobites and the earliest fossilized remains of these arthropods can be found in sedimentary rocks of the early Cambrian. Considering the aforesaid, that does not mean that there were none before.
It would appear that the early Cambrian was a period in the history of life in which certain arthropod taxa – among them trilobites – started to develop the first mineralised exoskeletons, a major precondition for allowing fossilization in the first place. All predecessors, which did not have any sort of hard outer shell, barely stood a chance of being preserved as fossils for more than 500 million years. And yet there are some exceptional places in which we may stumble upon the impressions of creatures likely to have been trilobite predecessors.
There are at least two convincing lines of argument to support the idea of trilobite predecessors in the Pre-Cambrian. The first line is based on the existence of trace fossils, the actual tracks left by marine animals while roaming on the sea floor (see illus. below), in sedimentary rocks of the Pre-Cambrian. These trace fossils are frequently found in layers clearly below the stratigraphic boundary to the Lower Cambrian, but containing no remains of the originator whatsoever. The same trace fossils, though, are evident in the fossil record throughout the entire Palaeozoic, now sometimes in association with actual trilobite remains.
The point is: Non-existing arthropods can not leave arthropod tracks behind. These trace fossils are petrified proof of the existence of at least some sort of arthropod way before the first development of a mineralised exoskeleton in the animal kingdom. Just as interesting are observations that these pre-Cambrian trace fossils seem to become the more complex, the closer they get to the Lower Cambrian boundary. It seems as if evolution had got its “show on the road” earlier than expected and was already in full swing when the “Cambrian Explosion” blasted all those nice trilobite fossils into existence. ;-)
The second line of argument, probably even more convincing, is based on the fossils of the Ediacaran, the last geological period of the Proterozoic eon, immediately preceding the Cambrian period. The Ediacaran biota was originally believed to represent the ancestral stem line of all the different groups of animals which “suddenly” appear in the fossil record of the early Cambrian. This thesis has been disputed insofar as some scientists are inclined to assign this fauna to an isolated and extinct animal kingdom with no relations whatsoever to any of the life forms appearing in the course of the Cambrian radiation.
Whatever the truth, the fossils of the soft-bodied Ediacaran biota appear merely as shallow impressions in sandstone. Their sometimes exceptionally good preservation is attributed to microbial mats which secreted sticky fluids or otherwise bound the sediment particles, allowing those impressions to stay undisturbed until covered by another protective layer of mud. With the evolution of grazing organisms in the Cambrian, this kind of preservation became less and less likely, and it is a fact that Ediacaran fossils are almost never found in beds that do not contain these microbial mats. Despite the arguments on the position of these fossils in the lineage of life, the Ediacaran fauna appears to include some taxa which, if not ancestors to the trilobita, at least seem to be at the threshold to claiming ancestry to the arthropoda, e.g. Vendia, Vendomia, Onega, Praecambridium, Parvancorina und Marywadea.
Each of the genera mentioned above shows a distinct separation of head and tail and a general morphology as could be expected of a primitive arthropod or its direct predecessor. There is even a taxon which by its overall construction seems to actually beg for assignment to a pre-Cambrian “trilobite” group.
Having said that, there is no use denying that we are still pretty much groping in the dark as far as the phylogenetic relationships of the trilobita are concerned. The fossil record is lacking in quantity, yet providing indications. Under exceptional circumstances as dominant in such places as Chenjiang and the Burgess Shale, entire faunal assemblages of non-calcified arthropods have been preserved in all their glory. Konservatlagerstätten dated to the Pre-Cambrian produced some candidates which by their overall characteristics are likely to be in some sort of relationship to the trilobites of the early Cambrian.
The most plausible scenario: Trilobites descended from pre-Cambrian ancestors which our Anglo-Saxon friends usually call “bilaterians”, meaning life forms with a “head” and a “tail” and a bilateral symmetry. More simple: they have a front and a back end, as well as an upside and downside (e.g. Spriggina floundersi). Whether we can already call these animals arthropods is a different question. But these pre-Cambrian taxa finally rose to become the arachnomorph arthropoda of the Cambrian, among which the trilobites appear to have played a predominant role.
No matter what the friends of a divine creation claim to be the truth: The fossil record contains convincing evidence that trilobites, as we know them from their fossilized remains, were preceded by trilobite-like arthropods which did not own a mineralised exoskeleton.
Trace fossils from the Pre-Cambrian indicate an arthropod evolution already under way when the “Cambrian Explosion” took place. Ediacaran soft-bodied faunas support the theory that primitive arthropods or their predecessors were alive and well in the late Proterozoic. Insisting on the “Cambrian Explosion” to be synonymous to the instant appearance of life is plainly wrong!
When Darwin talks about the „lowest Silurian stratum“ he is referring to rocks now classified as belonging to the Lower Cambrian. The Cambrian as a geological period was non-existent by the time of Darwin’s statement, although Sedgwick, in 1859, openly thought on further breaking down the Silurian which was then known as the oldest formation of the Palaeozoic. In essence, Darwin expresses his conviction that life was already abundant at a time of which we have no fossilized legacy.